1. Field of the Invention
The invention relates to a method and measuring instrument for measuring the water content of a web in the wire section of a paper machine.
2. Description of the Related Art
Accurate measuring of the water content of a paper web makes it possible to accurately adjust the paper machine and to make good quality paper. If the adjustment fails, the paper web becomes uneven in its properties and poor in quality. In the wire section in particular, it is possible to measure the amount of drained water as a machine direction (MD) function.
Water content is often measured by means of gamma radiation. In this measurement, the radiation source emits gamma radiation to the web, from which it scatters back to a detector or proceeds through the web to a detector. The density of the wire, the density of fibres and fillers and the like used in papermaking and the density of water affect the measuring result. Because the amount of water decreases as the web moves in the wire section, the effect of the water density on the measuring result diminishes accordingly. Thus, a change in the measuring result shows a change in the amount of water, i.e. water drainage in the wire section in machine direction.
However, this solution involves problems. Gamma radiation is ionising radiation, which is harmful to the people close to the measuring instrument. In addition, the area of influence of the radiation is so wide that using gamma radiation in close quarters is not sensible. Also, moving a gamma source from one place or country to another is difficult due to safety regulations, because a gamma source cannot be switched off.
Ultrasound technique has also been used in measuring water amounts and drainage. However, ultrasound measurement is not very accurate especially at low water contents, and air in the web also disturbs the measurement very much.